Joints of the musculoskeletal system of the human or animal body rely on the presence of healthy cartilaginous tissue for proper operation. Cartilaginous tissue can degenerate due to a number of causes, e.g., age or injury. Degradation of the tissue can reach a point where movement can cause severe discomfort and pain.
Degradation of tissue can occur in the spinal column. The spinal column comprises a series of 26 mobile vertebral bones, or vertebrae, connected by 75 stable articulations that control motion. The vertebrae are generally divided into posterior and anterior elements by thick pillows of bone called pedicles. The anterior element of the vertebra is a kidney shaped prism of bone with a concavity directed posteriorly and has flat superior and inferior surfaces called end plates. An intervertebral disc is sandwiched between adjacent pairs of vertebrae forming a joint between the adjacent pair of vertebrae. These discs are viscoelastic structures comprising a layer of strong, deformable, soft tissue. The intervertebral discs are subjected to a considerable variety of forces and moments resulting from the movements and loads of the spinal column. Each intervertebral disc has two components, being the annulus fibrosis surrounding a nucleus pulposus. The intervertebral disc cooperates with end plates of the vertebrae between which it is sandwiched.
The primary function of the nucleus pulposus of the disc is to give the disc its elasticity and compressibility characteristics to assist in sustaining and transmitting weight. The annulus fibrosis contains and limits the expansion of the nucleus pulposus during compression and also holds together successive vertebrae, resisting tension and torsion in the spine. The end plates of the vertebrae are responsible for the influx of nutrients into the disc and the efflux of waste products from within the disc.
With age or injury, a degenerative process of the disc may occur whereby its structures undergo morphological and biological changes affecting the efficiency with which the disc operates. Thus, the nucleus pulposus may reduce in volume and dehydrate resulting in a load reduction on the nucleus pulposus, a loss in intradiscal pressure and, hence, additional loading on the annulus fibrosis. In a normally functioning disc, the intradiscal pressure generated results in deformation of the end plates of the adjacent vertebrae generating the natural pumping action which assists in the influx of the nutrients and the efflux of waste products as stated above. A drop in intradiscal pressure therefore results in less end plate deformation. The nutrients supplied to the discal tissue are reduced and metabolic wastes are not removed with the same efficiency. This contributes to a degenerative cascade.
Radial and circumferential tears, cracks and fissures may begin to appear within the annulus fibrosis. If these defects do not heal, some of the nuclear material may begin to migrate into the defects in the annulus fibrosis. Migration of the nuclear material into the annulus fibrosis may cause stretching and delamination of layers of the annulus fibrosis resulting in back pain due to stimulation of the sinu-vertebral nerve. An intervertebral disc without a competent nucleus is unable to function properly. Further, since the spine is a cooperative system of elements, altering the structure and mechanics at one location of the spinal column may significantly increase stresses experienced at adjacent locations thereby further contributing to the degenerative cascade.
In the past, operative intervention has occurred to relieve lower back pain arising from intervertebral disc degeneration. Most of this operative intervention has been by way of a discectomy where leaking nuclear material is removed or, alternatively, fusion. The primary purpose of a discectomy is to excise any disc material that is impinging on the spinal nerve causing pain or sensory changes. Fusion means eliminating a motion segment between two vertebrae by use of a bone graft and sometimes internal fixation. Biomechanical studies show that fusion alters the biomechanics of the spine and causes increased stresses to be experienced at the junction between the fused and unfused segments. This promotes degeneration and begins the degenerative cycle anew. Clearly, being an invasive operative procedure, fusion is a risky procedure with no guarantee of success.
Due to the minimal success rate of these previous procedures, as well as their inability to restore complete function to the spinal column, alternative treatments have been sought in the form of artificial disc replacements. Theoretical advantages of artificial disc replacement over a fusion procedure include preservation or restoration of segmental motion in the spine, restoration of intervertebral architecture and foraminal height, sparing of adjacent segments of the spine from abnormal stresses and restoration of normal biomechanics across the lumbar spine. The established artificial disc replacement procedure consists of techniques that require a surgical incision on the abdomen, retraction of large blood vessels, a total excision of the anterior longitudinal ligament, anterior and posterior annulus along with the nucleus and near total removal of the lateral annulus and implantation of an articulated prosthesis. This is a major spinal column reconstruction operation carried out by a very invasive technique.
Accordingly, the present inventors have observed that there is a need for surgical procedures, methods, and/or systems which, as far as possible, restore the biomechanics of joints such as those between adjacent vertebrae of the spine by the provision of a tissue prosthesis mimicking natural, healthy cartilaginous tissue as well as a means of carrying out the surgical procedure in a minimally invasive manner and/or percutaneous manner.